The invention relates to an improved reactive power control for a static convertor, and, more particularly, to such a control including means to compensate for voltage transients caused by the connection and disconnection of reactance elements in an AC convertor network.
If a static convertor is used to provide an operational connection between a direct current network and an alternating current network, it is necessary to compensate for the variable consumption of reactive power by the convertor. Some prior art systems have compensated for this reactive power consumption by alternately connecting and disconnecting reactance elements, such as capacitors or reactors, on the AC side of the convertor. Such prior art systems have generally used step switches to incrementally connect or disconnect the reactance elements from the AC network. In addition, other systems have supplemented the reactive switching control scheme with apparatus for altering the angular control of a convertor in accordance with variations in reactive power.
For example, the U.S. Patent to Stackegard, U.S. Pat. No. 3,424,971, "Means for Controlling Reactive Power in an Invertor Station", discloses an apparatus that utilizes the angluar control of an invertor to supplement the reactive power compensation control of the switched reactance elements. The patent discloses apparatus to temporarily increase the margin of commutation of an invertor or increase the lower limit of the control angle of a rectifier in order to provide such supplemental reactive power control.
One problem of the prior art reactive power control methods is that they produce undesirable voltage transients when the reactance elements are connected or disconnected from the AC network. This voltage transient problem is particularly troublesome when the reactive elements are switched in a weak AC circuit.
Although it has been suggested that the voltage transient problem could be avoided by continuously controlling the reactive power of the network by means of a continuous angular control of the convertor, such a solution would require that the convertor would normally work with an unnecessarily great reactive power.
Accordingly, it is an object of the invention to provide a simple and effective means to compensate for voltage transients induced in an AC network without unduly increasing the operational reactive power of the convertor.
A further object of the invention is to provide an apparatus to compensate for voltage transients in an AC network by minimal adjustments to the angular control of the network convertor.
Another object of the invention is to provide an angular control apparatus that will momentarily adjust the angular control of the convertor to compensate for voltage transients when a reactance element is connected or disconnected from the AC network and will return the control angle to its original value after the transients have passed.
A further object of the invention is to compensate for a rapid change in the convertor control angle by either gradually building up the normal control angle to its maximum momentary compensation value or gradually reducing the maximum momentary value to the normal control angle value.
These and other objects of this invention will become apparent from a review of the detailed specification which follows and a consideration of the accompanying drawings.